This invention relates to the field of fuels, particularly liquid hydrocarbon fuel, and more particularly, high energy fuel for use in either jet or rocket propulsion. The invention more particularly pertains to high energy, high density synthetic fuels for use in missile and aircraft applications and is useful in ramjet, turbojet, and pulse jet engines. The invention is also applicable to rocket propulsion systems, i.e., those containing their own oxygen or oxidizing agent. This invention more particularly relates to an improved fuel for air launched ramjets having significantly lower viscosity, but substantially equivalent heating value than the presently used fuel known as RJ-5.
Fuels for military use and specifically United States Navy air launched ramjets must meet the requirements of high volumetric heating value, low viscosity, five year storage capability, excellent combustion characteristics, low toxicity, low cost, low freezing point, flash point no lower than 60.degree. C., compatibility with common materials, and absence of ionizable compounds. Heating value, viscosity, and cost are primary factors in choosing a fuel for ramjet application. A fuel having high viscosity is detrimental to the design and operation of air launched missiles since the missiles tend to become cold-soaked when during a mission they are carried at altitude prior to launch by an aircraft.
Present missile designs rely on a collapsible fuel cell whereby fuel pumping for delivery to the engine is accomplished through bleeding of high pressure engine exhaust gases to the annular space between the missile wall and the fuel cell. Unduly high fuel viscosity leads to inefficient operation of the fuel delivery system. Also, high viscosity fuel may fail to ignite in the ramjet engine leading to operational failure of the missile. Viscosity problems are further experienced when missiles are operated in cold climates where fuel viscosity accordingly increases.
Collapsible fuel cells are generally constructed of elastomeric materials. Fuels having components which tend to soften or otherwise attack the elastomeric fuel cells as well as seals, gaskets, or other fuel system components cannot be tolerated in air launched missile applications. This is particularly true since it is an established operational requirement that such missiles be capable of hermetic sealing and five year storage while in a fueled state, i.e., ready for launch. Failure of elastomeric cells could result in missile failure or explosion.
A prior art fuel, designated for military applications as RJ-5, and known as Shelldyne H, is more specifically described as a high density hydrocarbon fuel composed of perhydrodinorbornadiene, a hydrogenated dimerized norbonadiene prepared as described in U.S. Pat. No. 4,222,800 to Myers, Jr. et al. and references cited therein. RJ-5 is useful for ramjet air launch missile systems, but exhibits a viscosity high enough to limit the system's effectiveness in which it is employed. A fuel meeting other requirements, but exhibiting a lower viscosity under similar operational conditions would be highly desirable for the reasons outlined above.
The presently employed fuel for air launched ramjet application, designated as SI-80 which is formulated of 80 weight percent RJ-5 and 20 weight percent iso-butylbenzene, was developed to meet ramjet engine requirements while exhibiting a lower viscosity. SI-80 has a viscosity of 162 cps. at -40.degree. C., a flash point of 69.degree. C. and a heating value of 154,000 Btu/gal. The SI-80 formulation was chosen as the best available when considering its desirable viscosity, flash point, and energy content while meeting other requirements. A major disadvantage of SI-80 fuel is its tendency to attack elastomeric materials such as fuel cells and fuel system seals, etc., thus limiting its storability in a missile fuel system. Also, the toxicity of SI-80 to handling personnel, although found acceptable by the Navy, is of such a level as to bear improvement. This is thought to be attributable to the aromatic character of the isobutylbenzene component.
It has been suggested in the Myers, Jr. et al. patent that specific isomers or mixtures of specific isomers of hydrogenated norbornadiene dimers be blended with exo-tetrahydrodicyclopentadiene (exo-THDCP) designated JP-10, a known hydrocarbon fuel having unacceptable viscosities for the missile applications envisioned herein, for use as a fuel. These specific isomers, due to their high cost, are impractical as fuel components.